I thought Dr. Samuel Johnson created the first dictionary?

I agree.

Because languages change and evolve. New words are added. There would, at the very least, need to be updates done to Johnson's dictionary. But a major change in lexicography occurred at some point, changing a dictionary from prescriptive to reactive. Dictionaries are used (these days) to describe how language is used, taking word definitions from an aggregate of information gleaned from academic and lay sources. In Dr. Johnson's dictionary, he was writing down definitions of how a word "should" be used, not how it is actually used.

That shift necessitates new dictionaries.

There were dictionaries before the Oxford English Dictionary, but the OED was the most ambitious and comprehensive dictionary written up to that point, if not in the history of the language. Ever read any of its entries? it not only defines the words and gives examples of use, it traces the history of each and every word!

There's an online version now, requires a subscription or a library card.